Plasma represents a specific aggregate condition produced from a gas. Every gas in principle comprises atoms and/or molecules. In a plasma the main part of the gas is ionized. This means that a supply of energy will split the atoms or molecules into positive and negative charge carriers, namely into ions and electrons. Plasma is suitable for processing work pieces, as the electrically charged particles are chemically reactive to a high degree and can also be influenced by electric fields. The charged particles can be accelerated onto an object by an electric field, where they can separate individual atoms from the same upon impact. The separated atoms can be transported away by a gas flow (etching) or stored on other objects as a coating (production of thin films). Such processes are applied by a plasma in particular when extremely thin layers, in particular within a range of a few atom layers, are to be processed. Typical applications are semi-conductor technologies (coating, etching, etc.), flat screen monitors (similar to semi-conductor technology), solar cells (similar to semi-conductor technology), architectural glass coatings (heat protection, glare protection, etc.) storage media (CD, DVD, hard drives), decorative layers (colored glass, etc.), and tool hardening. These applications place high requirements on accuracy and process stability. Plasma can also serve for the excitation of lasers.
In order to generate a plasma from a gas, the gas needs to be supplied with energy. This can be realized in different ways, for example by light, heat, and/or electric energy. A plasma for processing work pieces is typically ignited in a plasma chamber and maintained there. For this, an inert gas, for example argon, is normally introduced into the plasma chamber at low pressure. The gas is then exposed to an electric field via electrodes and/or antennae. Plasma is created or ignited when several conditions are fulfilled. A small number of free charge carriers must first be present, whereby free electrons that are usually present to a very small extent are being used. The free charge carriers are accelerated so strongly by the electric field that they separate further electrons upon impact with atoms or molecules of the inert gas, which creates further positively charged ions and further negatively charged electrons. The additional free charge carriers are in turn accelerated and create further ions and electrons upon impact. This will initiate an avalanche effect. The continuous production of ions and electrons is counteracted by the discharges during the collision of these particles with the wall of the plasma chamber or other objects as well as by natural recombination, i.e., electrons are attracted by ions and recombine to form electrically neutral atoms or molecules. Once a plasma has been ignited, it must therefore be continuously supplied with energy in order to maintain the same.
This energy supply can be realized via a direct current (DC) or an alternating current (AC) power supply. The frequencies that occur during plasma excitation with an AC power supply can reach gigahertz ranges.
Short-term or longer-term flashovers, so-called arcs, can occur in the plasma, which are undesirable. When such an arc is detected it must be ensured that the arc is extinguished as quickly as possible, i.e., that it cannot develop fully.
It is known to switch off the energy supply completely upon detection of an arc in order to enforce the extinguishing of the arc. One disadvantage of this approach is that the plasma process is interrupted and that it takes a certain time before the plasma can be reignited after extinguishing the arc and plasma processing can be continued. It is also often not possible to ascertain for definite whether an arc has been extinguished. Appropriately long periods are therefore envisaged, after which an arc will normally be extinguished. This will however lead to longer interruptions of the plasma processing process.